U.S. patent application number 14/482423 was filed with the patent office on 2016-03-10 for system and methods for fuel system leak detection.
The applicant listed for this patent is Ford Global Technologies, LLC. Invention is credited to Aed M. Dudar, Robert Roy Jentz, Pankaj Kumar, Imad Hassan Makki, Fling Finn Tseng.
Application Number | 20160069771 14/482423 |
Document ID | / |
Family ID | 55437257 |
Filed Date | 2016-03-10 |
United States Patent
Application |
20160069771 |
Kind Code |
A1 |
Makki; Imad Hassan ; et
al. |
March 10, 2016 |
SYSTEM AND METHODS FOR FUEL SYSTEM LEAK DETECTION
Abstract
A method for a fuel system comprises indicating a leak in the
fuel system based on a pressure change rate distribution during a
first condition including a sealed fuel system and a pressure
change rate within a threshold of zero. By indicating a leak based
on a pressure change rate distribution rather than through simple
thresholding, an engine-off natural vacuum test may be performed in
a greater range of conditions. In this way, the execution rate of
the test may be increased while maintaining or improving the leak
detection rate and reducing the misclassification rate.
Inventors: |
Makki; Imad Hassan;
(Dearborn Heights, MI) ; Tseng; Fling Finn; (Ann
Arbor, MI) ; Dudar; Aed M.; (Canton, MI) ;
Jentz; Robert Roy; (Westland, MI) ; Kumar;
Pankaj; (Dearborn, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ford Global Technologies, LLC |
Dearborn |
MI |
US |
|
|
Family ID: |
55437257 |
Appl. No.: |
14/482423 |
Filed: |
September 10, 2014 |
Current U.S.
Class: |
73/40.5R |
Current CPC
Class: |
F02M 25/0809
20130101 |
International
Class: |
G01M 3/26 20060101
G01M003/26 |
Claims
1. A method for a fuel system, comprising: indicating a leak in the
fuel system based on a pressure change rate distribution during a
first condition including a sealed fuel system and a pressure
change rate within a threshold of zero.
2. The method of claim 1, further comprising: applying a low-pass
filter to the pressure change rate distribution; and then
indicating the leak in the fuel system based on the filtered
pressure change rate distribution.
3. The method of claim 1, further comprising: indicating the leak
in the fuel system based on a minimum error between a measured fuel
tank pressure and a predicted fuel tank pressure.
4. The method of claim 3, where the measured fuel tank pressure
comprises an equilibrium fuel tank pressure.
5. The method of claim 3, where the predicted fuel tank pressure is
based on a fuel tank temperature.
6. The method of claim 5, where the predicted fuel tank pressure is
based on a fuel composition.
7. The method of claim 6, where the predicted fuel tank pressure is
determined using an Antoine equation relationship.
8. The method of claim 3, further comprising: classifying the
pressure change rate distribution and minimum error using a linear
classifier; and indicating the leak in the fuel system based on an
output of the linear classifier.
9. The method of claim 8, further comprising: indicating an
indeterminate result if the output of the linear classifier is
within a threshold of a decision boundary.
10. A method for a fuel system, comprising: sealing a fuel system;
and executing an evaporative emissions leak test only if a duration
of a fuel system pressure increase event is greater than a
threshold.
11. The method of claim 10, where executing an evaporative
emissions leak test further comprises: extracting one or more fuel
system pressure features for classification via a linear
classifier; and indicating a fuel system leak based on the
extracted fuel system pressure features.
12. The method of claim 11, where the one or more fuel system
pressure features include a minimum error between a measured fuel
tank pressure and a predicted fuel tank pressure.
13. The method of claim 12, where the measured fuel tank pressure
comprises an equilibrium fuel tank pressure.
14. The method of claim 12, where the predicted fuel tank pressure
is based on a fuel tank temperature.
15. The method of claim 14, where the predicted fuel tank pressure
is based on a fuel composition.
16. The method of claim 15, where the predicted fuel tank pressure
is determined using an Antoine equation relationship.
17. The method of claim 11, where the one or more fuel system
pressure features include a pressure change rate distribution
within the fuel system during a condition where a pressure change
rate is within a threshold of zero.
18. The method of claim 17, where the pressure change rate
distribution is filtered using a low-pass filter prior to
classification by the linear classifier.
19. The method of claim 11, further comprising: comparing an output
of the linear classifier to a decision boundary; and indicating an
indeterminate result if the output of the linear classifier is
within a threshold of a decision boundary.
20. A fuel system for a vehicle, comprising: a fuel tank coupled to
an evaporative emissions system; a valve coupled between the fuel
tank and atmosphere; a pressure sensor coupled between the fuel
tank and the valve; and a controller holding executable
instructions in non-transitory memory, that when executed, cause
the controller to: close the valve; execute an evaporative
emissions leak test responsive to a duration of a fuel system
pressure increase event being greater than a threshold; determine a
minimum error between an equilibrium fuel tank pressure and a
predicted fuel tank pressure; determine a pressure change rate
distribution within the fuel system during a condition where a
pressure change rate is within a threshold of zero; using a linear
classifier, indicate a fuel system leak based on the determined
minimum error and the determined pressure change rate distribution;
and indicate an indeterminate result if an output of the linear
classifier is within a threshold of a decision boundary.
Description
BACKGROUND AND SUMMARY
[0001] Vehicle emission control systems may be configured to store
fuel vapors from fuel tank refueling and diurnal engine operations,
and then purge the stored vapors during a subsequent engine
operation. In an effort to meet stringent federal emissions
regulations, emission control systems may need to be intermittently
diagnosed for the presence of leaks that could release fuel vapors
to the atmosphere.
[0002] Evaporative leaks may be identified using engine-off natural
vacuum (EONV) during conditions when a vehicle engine is not
operating. In particular, a fuel system may be isolated at an
engine-off event. The pressure in such a fuel system will increase
if the tank is heated further (e.g. from hot exhaust or a hot
parking surface) as liquid fuel vaporizes. As a fuel tank cools
down, a vacuum is generated therein as fuel vapors condense to
liquid fuel. Vacuum generation is monitored and leaks identified
based on expected vacuum development or expected rates of vacuum
development.
[0003] Federal requirements for leak detection progressively become
more stringent. In particular, manufacturers desiring to designate
vehicles as practically zero emission vehicles (PZEVs) must prove
high levels of performance both for individual vehicles and
manufacturing runs of vehicles. Vehicles are required to perform
leak tests with a high rate of execution, a low threshold for
detection, and a high robustness. Typically, on-board sensors are
used to identify situations where leak tests are likely to result
in a definitive result (either pass or fail). Relaxing entry
conditions to maintain a high rate of execution may reduce the
robustness of a test, as EONV tests may have to be performed under
less than ideal conditions.
[0004] The inventors herein have recognized the above issues and
have developed systems and methods to at least partially address
them. In one example, a method for a fuel system comprises
indicating a leak in the fuel system based on a pressure change
rate distribution during a first condition including a sealed fuel
system and a pressure change rate within a threshold of zero. By
indicating a leak based on a pressure change rate distribution
rather than through simple thresholding, an engine-off natural
vacuum test may be performed in a greater range of conditions. In
this way, the execution rate of the test may be increased while
maintaining or improving the leak detection rate and reducing the
misclassification rate.
[0005] In another example, a method for a fuel system comprises
sealing a fuel system; and executing an evaporative emissions leak
test only if a duration of a fuel system pressure increase event is
greater than a threshold. By evaluating the initial fuel system
pressure increase event, the misclassification rate of an
engine-off natural vacuum test may be reduced, thereby increasing
the robustness of the test. In this way, false results can be
avoided, saving unnecessary warranty inspections and reducing
overall costs for the vehicle manufacturer.
[0006] In yet another example, a fuel system for a vehicle,
comprising a fuel tank coupled to an evaporative emissions system;
a valve coupled between the fuel tank and atmosphere; a pressure
sensor coupled between the fuel tank and the valve; and a
controller holding executable instructions in non-transitory
memory, that when executed, cause the controller to: close the
valve; execute an evaporative emissions leak test responsive to a
duration of a fuel system pressure increase event being greater
than a threshold; determine a minimum error between an equilibrium
fuel tank pressure and a predicted fuel tank pressure; determine a
pressure change rate distribution within the fuel system during a
condition where a pressure change rate is within a threshold of
zero; using a linear classifier, indicate a fuel system leak based
on the determined minimum error and the determined pressure change
rate distribution; and indicate an indeterminate result if an
output of the linear classifier is within a threshold of a decision
boundary. By extracting fuel system features for classification by
a linear classifier, the EONV test may provide enough extractable
features to classify a fuel system as intact or leaky without
running the test to a vacuum threshold endpoint. This, in turn may
reduce the amount of battery power used in performing the test.
[0007] The above advantages and other advantages, and features of
the present description will be readily apparent from the following
Detailed Description when taken alone or in connection with the
accompanying drawings.
[0008] It should be understood that the summary above is provided
to introduce in simplified form a selection of concepts that are
further described in the detailed description. It is not meant to
identify key or essential features of the claimed subject matter,
the scope of which is defined uniquely by the claims that follow
the detailed description. Furthermore, the claimed subject matter
is not limited to implementations that solve any disadvantages
noted above or in any part of this disclosure.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0009] FIG. 1 schematically shows a fuel system and an emissions
system for an example vehicle engine.
[0010] FIG. 2 shows a flow-chart for a high-level method for an
engine-off natural vacuum test.
[0011] FIG. 3A shows a flow-chart for a high-level method for
comparing observed vapor pressure to expected vapor pressure.
[0012] FIG. 3B shows an example plot of pressure/temperature
relationships for two different fuel compositions.
[0013] FIG. 3C shows a timeline for an example engine-off natural
vacuum test.
[0014] FIG. 4 shows a flow-chart for a high-level method for
determining pressure change rate distribution content for a sealed
system.
[0015] FIG. 5 shows an example plot of classified data sets
representative of intact fuel systems and leaky fuel systems.
DETAILED DESCRIPTION
[0016] This detailed description relates to systems and methods for
a vehicle fuel system. More specifically, the description relates
to systems and methods for performing engine-off natural vacuum
tests on a fuel system for the purpose of detecting leaks. The fuel
system may be included in a hybrid vehicle system, as shown
schematically in FIG. 1. FIG. 2 shows an example method for an EONV
test, including the extraction of fuel system pressure features
that may be classified in order to determine whether the fuel
system is leaky or intact. One example fuel system pressure feature
is a comparison of observed vapor pressure and expected vapor
pressure based on the Antoine equation. An example method thereof
is shown in FIG. 3A. FIG. 3B shows an example plot of
pressure/temperature relationships for two different fuel
compositions, the pressure/temperature relationships transformed by
the Antoine equation. An example timeline for an EONV test
including this comparison is shown in FIG. 3C. Another example fuel
system pressure feature includes determining pressure change rate
distribution content for a sealed fuel system, as shown in FIG. 4A.
The extracted features may be classified using a linear classifier.
The linear classifier may classify data sets within a threshold of
a decision boundary as indeterminate, as shown in FIG. 5.
[0017] FIG. 1 shows a schematic depiction of a hybrid vehicle
system 6 that can derive propulsion power from engine system 8
and/or an on-board energy storage device, such as a battery system
(not shown). An energy conversion device, such as a generator (not
shown), may be operated to absorb energy from vehicle motion and/or
engine operation, and then convert the absorbed energy to an energy
form suitable for storage by the energy storage device.
[0018] Engine system 8 may include an engine 10 having a plurality
of cylinders 30. Engine 10 includes an engine intake 23 and an
engine exhaust 25. Engine intake 23 includes an air intake throttle
62 fluidly coupled to the engine intake manifold 44 via an intake
passage 42. Air may enter intake passage 42 via air filter 52.
Engine exhaust 25 includes an exhaust manifold 48 leading to an
exhaust passage 35 that routes exhaust gas to the atmosphere.
Engine exhaust 25 may include one or more emission control devices
70 mounted in a close-coupled position. The one or more emission
control devices may include a three-way catalyst, lean NOx trap,
diesel particulate filter, oxidation catalyst, etc. It will be
appreciated that other components may be included in the engine
such as a variety of valves and sensors, as further elaborated in
herein. In some embodiments, wherein engine system 8 is a boosted
engine system, the engine system may further include a boosting
device, such as a turbocharger (not shown).
[0019] Engine system 8 is coupled to a fuel system 18. Fuel system
18 includes a fuel tank 20 coupled to a fuel pump 21 and a fuel
vapor canister 22. During a fuel tank refueling event, fuel may be
pumped into the vehicle from an external source through refueling
port 108. Fuel tank 20 may hold a plurality of fuel blends,
including fuel with a range of alcohol concentrations, such as
various gasoline-ethanol blends, including E10, E85, gasoline,
etc., and combinations thereof. A fuel level sensor 106 located in
fuel tank 20 may provide an indication of the fuel level ("Fuel
Level Input") to controller 12. As depicted, fuel level sensor 106
may comprise a float connected to a variable resistor.
Alternatively, other types of fuel level sensors may be used.
[0020] Fuel pump 21 is configured to pressurize fuel delivered to
the injectors of engine 10, such as example injector 66. While only
a single injector 66 is shown, additional injectors are provided
for each cylinder. It will be appreciated that fuel system 18 may
be a return-less fuel system, a return fuel system, or various
other types of fuel system. Vapors generated in fuel tank 20 may be
routed to fuel vapor canister 22, via conduit 31, before being
purged to the engine intake 23.
[0021] Fuel vapor canister 22 is filled with an appropriate
adsorbent for temporarily trapping fuel vapors (including vaporized
hydrocarbons) generated during fuel tank refueling operations, as
well as diurnal vapors. In one example, the adsorbent used is
activated charcoal. When purging conditions are met, such as when
the canister is saturated, vapors stored in fuel vapor canister 22
may be purged to engine intake 23 by opening canister purge valve
112. While a single canister 22 is shown, it will be appreciated
that fuel system 18 may include any number of canisters. In one
example, canister purge valve 112 may be a solenoid valve wherein
opening or closing of the valve is performed via actuation of a
canister purge solenoid.
[0022] Canister 22 may include a buffer 22a (or buffer region),
each of the canister and the buffer comprising the adsorbent. As
shown, the volume of buffer 22a may be smaller than (e.g., a
fraction of) the volume of canister 22. The adsorbent in the buffer
22a may be same as, or different from, the adsorbent in the
canister (e.g., both may include charcoal). Buffer 22a may be
positioned within canister 22 such that during canister loading,
fuel tank vapors are first adsorbed within the buffer, and then
when the buffer is saturated, further fuel tank vapors are adsorbed
in the canister. In comparison, during canister purging, fuel
vapors are first desorbed from the canister (e.g., to a threshold
amount) before being desorbed from the buffer. In other words,
loading and unloading of the buffer is not linear with the loading
and unloading of the canister. As such, the effect of the canister
buffer is to dampen any fuel vapor spikes flowing from the fuel
tank to the canister, thereby reducing the possibility of any fuel
vapor spikes going to the engine.
[0023] Canister 22 includes a vent 27 for routing gases out of the
canister 22 to the atmosphere when storing, or trapping, fuel
vapors from fuel tank 20. Vent 27 may also allow fresh air to be
drawn into fuel vapor canister 22 when purging stored fuel vapors
to engine intake 23 via purge line 28 and purge valve 112. While
this example shows vent 27 communicating with fresh, unheated air,
various modifications may also be used. Vent 27 may include a
canister vent valve 114 to adjust a flow of air and vapors between
canister 22 and the atmosphere. The canister vent valve may also be
used for diagnostic routines. When included, the vent valve may be
opened during fuel vapor storing operations (for example, during
fuel tank refueling and while the engine is not running) so that
air, stripped of fuel vapor after having passed through the
canister, can be pushed out to the atmosphere. Likewise, during
purging operations (for example, during canister regeneration and
while the engine is running), the vent valve may be opened to allow
a flow of fresh air to strip the fuel vapors stored in the
canister. In one example, canister vent valve 114 may be a solenoid
valve wherein opening or closing of the valve is performed via
actuation of a canister vent solenoid. In particular, the canister
vent valve may be an open that is closed upon actuation of the
canister vent solenoid. In some examples, an air filter may be
coupled in vent 27 between canister vent valve 114 and
atmosphere.
[0024] As such, hybrid vehicle system 6 may have reduced engine
operation times due to the vehicle being powered by engine system 8
during some conditions, and by the energy storage device under
other conditions. While the reduced engine operation times reduce
overall carbon emissions from the vehicle, they may also lead to
insufficient purging of fuel vapors from the vehicle's emission
control system. To address this, a fuel tank isolation valve 110
may be optionally included in conduit 31 such that fuel tank 20 is
coupled to canister 22 via the valve. During regular engine
operation, isolation valve 110 may be kept closed to limit the
amount of diurnal or "running loss" vapors directed to canister 22
from fuel tank 20. During refueling operations, and selected
purging conditions, isolation valve 110 may be temporarily opened,
e.g., for a duration, to direct fuel vapors from the fuel tank 20
to canister 22. By opening the valve during purging conditions when
the fuel tank pressure is higher than a threshold (e.g., above a
mechanical pressure limit of the fuel tank above which the fuel
tank and other fuel system components may incur mechanical damage),
the refueling vapors may be released into the canister and the fuel
tank pressure may be maintained below pressure limits. While the
depicted example shows isolation valve 110 positioned along conduit
31, in alternate embodiments, the isolation valve may be mounted on
fuel tank 20. The fuel system may be considered to be sealed when
isolation valve 110 is closed. In embodiments where the fuel system
does not include isolation valve 110, the fuel system may be
considered sealed when purge valve 112 and canister vent valve 114
are both closed.
[0025] One or more pressure sensors 120 may be coupled to fuel
system 18 for providing an estimate of a fuel system pressure. In
one example, the fuel system pressure is a fuel tank pressure,
wherein pressure sensor 120 is a fuel tank pressure sensor coupled
to fuel tank 20 for estimating a fuel tank pressure or vacuum
level. While the depicted example shows pressure sensor 120
directly coupled to fuel tank 20, in alternate embodiments, the
pressure sensor may be coupled between the fuel tank and canister
22, specifically between the fuel tank and isolation valve 110. In
still other embodiments, a first pressure sensor may be positioned
upstream of the isolation valve (between the isolation valve and
the canister) while a second pressure sensor is positioned
downstream of the isolation valve (between the isolation valve and
the fuel tank), to provide an estimate of a pressure difference
across the valve. In some examples, a vehicle control system may
infer and indicate a fuel system leak based on changes in a fuel
tank pressure during a leak diagnostic routine.
[0026] One or more temperature sensors 121 may also be coupled to
fuel system 18 for providing an estimate of a fuel system
temperature. In one example, the fuel system temperature is a fuel
tank temperature, wherein temperature sensor 121 is a fuel tank
temperature sensor coupled to fuel tank 20 for estimating a fuel
tank temperature. While the depicted example shows temperature
sensor 121 directly coupled to fuel tank 20, in alternate
embodiments, the temperature sensor may be coupled between the fuel
tank and canister 22.
[0027] Fuel vapors released from canister 22, for example during a
purging operation, may be directed into engine intake manifold 44
via purge line 28. The flow of vapors along purge line 28 may be
regulated by canister purge valve 112, coupled between the fuel
vapor canister and the engine intake. The quantity and rate of
vapors released by the canister purge valve may be determined by
the duty cycle of an associated canister purge valve solenoid (not
shown). As such, the duty cycle of the canister purge valve
solenoid may be determined by the vehicle's powertrain control
module (PCM), such as controller 12, responsive to engine operating
conditions, including, for example, engine speed-load conditions,
an air-fuel ratio, a canister load, etc. By commanding the canister
purge valve to be closed, the controller may seal the fuel vapor
recovery system from the engine intake. An optional canister check
valve (not shown) may be included in purge line 28 to prevent
intake manifold pressure from flowing gases in the opposite
direction of the purge flow. As such, the check valve may be
necessary if the canister purge valve control is not accurately
timed or the canister purge valve itself can be forced open by a
high intake manifold pressure. An estimate of the manifold absolute
pressure (MAP) or manifold vacuum (ManVac) may be obtained from MAP
sensor 118 coupled to intake manifold 44, and communicated with
controller 12. Alternatively, MAP may be inferred from alternate
engine operating conditions, such as mass air flow (MAF), as
measured by a MAF sensor (not shown) coupled to the intake
manifold.
[0028] Fuel system 18 may be operated by controller 12 in a
plurality of modes by selective adjustment of the various valves
and solenoids. For example, the fuel system may be operated in a
fuel vapor storage mode (e.g., during a fuel tank refueling
operation and with the engine not running), wherein the controller
12 may open isolation valve 110 and canister vent valve 114 while
closing canister purge valve (CPV) 112 to direct refueling vapors
into canister 22 while preventing fuel vapors from being directed
into the intake manifold.
[0029] As another example, the fuel system may be operated in a
refueling mode (e.g., when fuel tank refueling is requested by a
vehicle operator), wherein the controller 12 may open isolation
valve 110 and canister vent valve 114, while maintaining canister
purge valve 112 closed, to depressurize the fuel tank before
allowing enabling fuel to be added therein. As such, isolation
valve 110 may be kept open during the refueling operation to allow
refueling vapors to be stored in the canister. After refueling is
completed, the isolation valve may be closed.
[0030] As yet another example, the fuel system may be operated in a
canister purging mode (e.g., after an emission control device
light-off temperature has been attained and with the engine
running), wherein the controller 12 may open canister purge valve
112 and canister vent valve while closing isolation valve 110.
Herein, the vacuum generated by the intake manifold of the
operating engine may be used to draw fresh air through vent 27 and
through fuel vapor canister 22 to purge the stored fuel vapors into
intake manifold 44. In this mode, the purged fuel vapors from the
canister are combusted in the engine. The purging may be continued
until the stored fuel vapor amount in the canister is below a
threshold. During purging, the learned vapor amount/concentration
can be used to determine the amount of fuel vapors stored in the
canister, and then during a later portion of the purging operation
(when the canister is sufficiently purged or empty), the learned
vapor amount/concentration can be used to estimate a loading state
of the fuel vapor canister.
[0031] Vehicle system 6 may further include control system 14.
Control system 14 is shown receiving information from a plurality
of sensors 16 (various examples of which are described herein) and
sending control signals to a plurality of actuators 81 (various
examples of which are described herein). As one example, sensors 16
may include exhaust gas sensor 126 located upstream of the emission
control device, temperature sensor 128, MAP sensor 118, pressure
sensor 120, and pressure sensor 129. Other sensors such as
additional pressure, temperature, air/fuel ratio, and composition
sensors may be coupled to various locations in the vehicle system
6. For example, ambient temperature and pressure sensors may be
coupled to the exterior of the vehicle body. As another example,
the actuators may include fuel injector 66, isolation valve 110,
purge valve 112, vent valve 114, fuel pump 21, and throttle 62.
[0032] Control system 14 may further receive information regarding
the location of the vehicle from an on-board global positioning
system (GPS). Information received from the GPS may include vehicle
speed, vehicle altitude, vehicle position, etc. This information
may be used to infer engine operating parameters, such as local
barometric pressure. Control system 14 may further be configured to
receive information via the internet or other communication
networks. Information received from the GPS may be cross-referenced
to information available via the internet to determine local
weather conditions, local vehicle regulations, etc. Control system
14 may use the internet to obtain updated software modules which
may be stored in non-transitory memory.
[0033] The control system 14 may include a controller 12.
Controller 12 may be configured as a conventional microcomputer
including a microprocessor unit, input/output ports, read-only
memory, random access memory, keep alive memory, a controller area
network (CAN) bus, etc. Controller 12 may be configured as a
powertrain control module (PCM). The controller may be shifted
between sleep and wake-up modes for additional energy efficiency.
The controller may receive input data from the various sensors,
process the input data, and trigger the actuators in response to
the processed input data based on instruction or code programmed
therein corresponding to one or more routines. Example control
routines are described herein with regard to FIGS. 2, 3A, and
4.
[0034] Controller 12 may also be configured to intermittently
perform leak detection routines on fuel system 18 (e.g., fuel vapor
recovery system) to confirm that the fuel system is not degraded.
As such, various diagnostic leak detection tests may be performed
while the engine is off (engine-off leak test) or while the engine
is running (engine-on leak test). Leak tests performed while the
engine is running may include applying a negative pressure on the
fuel system for a duration (e.g., until a target fuel tank vacuum
is reached) and then sealing the fuel system while monitoring a
change in fuel tank pressure (e.g., a rate of change in the vacuum
level, or a final pressure value). Leak tests performed while the
engine is not running may include sealing the fuel system following
engine shut-off and monitoring a change in fuel tank pressure. This
type of leak test is referred to herein as an engine-off natural
vacuum test (EONV). In sealing the fuel system following engine
shut-off, a vacuum will develop in the fuel tank as the tank cools
and fuel vapors are condensed to liquid fuel. The amount of vacuum
and/or the rate of vacuum development may be compared to expected
values that would occur for a system with no leaks, and/or for a
system with leaks of a predetermined size. Following a vehicle-off
event, as heat continues to be rejected from the engine into the
fuel tank, the fuel tank pressure will initially rise. During
conditions of relatively high ambient temperature, a pressure build
above a threshold may be considered a passing test.
[0035] Federal requirements for leak detection progressively become
more stringent. In particular, manufacturers desiring to designate
vehicles as practically zero emission vehicles (PZEVs) must prove
high levels of performance both for individual vehicles and
manufacturing runs of vehicles. Vehicles are required to perform
leak tests with a high rate of execution, with a low threshold for
detection, and with a high robustness. Typically, on-board sensors
are used to identify situations where leak tests are likely to
result in a definitive result (either pass or fail). Relaxing entry
conditions to maintain a high rate of execution may reduce the
robustness of a test, as EONV tests may have to be performed under
less than ideal conditions.
[0036] FIG. 2 shows a flow chart for an example high-level method
200 for an engine-off natural vacuum test for a vehicle fuel
system. Method 200 will be described in relation to the hybrid
vehicle system depicted in FIG. 1, but it should be understood that
similar methods may be used with other systems without departing
from the scope of this disclosure. Method 200 may be carried out by
a controller, such as controller 12, and may be stored as
executable instructions in non-transitory memory.
[0037] Method 200 begins at 205. At 205, method 200 includes
evaluating operating conditions. Operating conditions may be
measured, estimated, or inferred. Among other conditions, operating
conditions may include various vehicle conditions, such as vehicle
speed, vehicle location, etc., various engine conditions, such as
engine status, engine load, engine temperature, etc., various fuel
system conditions, such as fuel tank pressure, fuel tank
temperature, fuel fill level, fuel vapor canister level, etc., and
various ambient conditions, such as ambient temperature, humidity,
barometric pressure, etc.
[0038] Continuing at 210, method 200 includes determining whether
entry conditions are met for an EONV test. For an engine-off
natural vacuum test, the engine must be at rest with all cylinders
off. In some examples, the vehicle must be shut off completely.
Entry conditions may include an indication to perform an EONV test
based on a number of driving trips since the most recent EONV test.
Additional entry conditions may include a threshold amount of time
passed since the previous EONV test, a threshold length of engine
run time prior to the engine-off event, a threshold air mass
passing through an engine intake during the engine run time prior
to the engine-off event, a threshold amount of fuel in the fuel
tank, a threshold ambient barometric pressure, an ambient
temperature within a range of ambient temperatures, and a threshold
battery state of charge. If entry conditions are not met, method
200 may proceed to 215. At 215, method 200 may include recording
that an EONV test was not executed, and may further include setting
a flag to retry the EONV test at the next detected vehicle-off
event. Method 200 may then end.
[0039] Although entry conditions may be met initially during the
execution of method 200, systemic and ambient conditions may change
during the execution of the method. For example, an engine restart
or refueling event may be sufficient to abort the method at any
point prior to completing method 200. If such events are detected
that would interfere with the execution of method 200 or the
interpretation of results derived from executing method 200, method
200 may proceed to 215, record that an EONV test was not executed,
set a flag to retry the EONV test at the next detected vehicle-off
event, and then end.
[0040] If entry conditions for an EONV test are met, method 200 may
proceed to 220. At 220, method 200 may include sealing the vehicle
fuel system from atmosphere. For the hybrid vehicle system depicted
in FIG. 1, this may include closing (or maintaining closed) CPV
112, and may further include closing CVV 114. Additionally or
alternatively, in some examples, FTIV 110 may be closed. Continuing
at 225, method 200 may include monitoring pressure within the
sealed fuel system. Fuel system pressure may be inferred or
measured, for example using fuel tank pressure sensor 120.
[0041] Continuing at 230, method 200 may optionally include
unsealing the fuel system, allowing the fuel system pressure to
equilibrate, and then re-sealing the fuel system. For example,
following the initial sealing of the fuel system, the fuel system
pressure may increase as heat is rejected to the fuel tank from the
engine. In some examples, the fuel tank may be initially sealed for
a duration, and then unsealed if the fuel system pressure fails to
reach a threshold pressure by the end of the duration. The fuel
system may then be allowed to equilibrate to atmospheric pressure,
and resealed to allow a vacuum to develop in the fuel system as the
fuel tank continues to cool. In some examples, the fuel system may
be unsealed upon the fuel system pressure reaching a threshold,
then resealed to allow for a second pressure-rise event to occur.
However, in some examples, upon the fuel system reaching a
threshold pressure following the first system sealing, the fuel
system may be unsealed, but not resealed. During time periods where
the fuel system is sealed, the pressure may be monitored as
described above. Pressure may also be monitored during time periods
where the fuel system is unsealed, but this data may not contribute
to the determination of fuel system integrity.
[0042] At 235, method 200 may include determining whether the fuel
system pressure rise upon initially sealing the fuel system is
greater than a threshold. The threshold may be predetermined or may
be based on current operating conditions. The initial pressure rise
may be characterized as a change in pressure over a predetermined
duration, and/or as an amount of time the fuel system takes to
reach a threshold pressure. For example, a duration of a fuel
system pressure increase event may be monitored to determine
whether the duration is greater than a threshold. Failure to meet
the threshold pressure change may indicate a gross leak, or may
indicate that conditions are not met for performing a conclusive
EONV test. For example, the fuel system pressure may not initially
increase if a gas cap is unsealed, or if ambient temperature is
similar to fuel system temperature. If the initial pressure rise is
less than the threshold, method 200 may proceed to 215. At 215,
method 200 may include recording that an EONV test was not
executed, and may further include setting a flag to retry the EONV
test at the next detected vehicle-off event. Method 200 may then
end.
[0043] If the initial pressure rise is greater than the threshold,
method 200 may proceed to 240. At 240, method 200 may include
extracting fuel system pressure features for classification. The
fuel system pressure features may be derived from the monitored
pressure within the sealed fuel system as described herein, and may
be based on absolute pressure values and/or rates of change of
pressure values over time. The pressure features may be further
based on systemic or ambient conditions. One or more fuel system
pressure features may be extracted. Herein, three fuel system
pressure features are described, but other fuel system pressure
features may be derived in addition to or as an alternative to the
features described.
[0044] For example, at 241, method 200 may include comparing one or
more resulting fuel system pressures to one or more threshold
pressures. For example, method 200 may include a pressure rise test
portion where an initial fuel tank pressure increase is compared to
a threshold pressure, and/or a vacuum test portion where a fuel
tank vacuum is compared to a threshold vacuum. While the engine is
still cooling down post shut-down, there may be additional heat
rejected to the fuel tank. With the fuel system sealed via the
closing of the CVV, the pressure in the fuel system may rise due to
fuel volatizing with increased temperature. The pressure rise test
portion may include monitoring fuel tank pressure for a period of
time. Fuel tank pressure may be monitored until the pressure
reaches a threshold pressure, the threshold pressure indicative of
no leaks above a threshold size in the fuel system. In some
examples, the rate of pressure change may be compared to an
expected rate of pressure change.
[0045] As described above, following the initial pressure rise, the
fuel system may be unsealed, allowed to equilibrate, and resealed.
As the fuel tank cools, the fuel vapors condense into liquid fuel,
creating a vacuum within the sealed tank. Fuel tank pressure may be
monitored until the vacuum reaches a predetermined threshold
vacuum, the predetermined threshold vacuum indicative of no leaks
above a threshold size in the fuel tank. In some examples, the rate
of pressure change may be compared to an expected rate of pressure
change. The threshold pressure(s) and vacuum(s) may be based on
system and ambient conditions, such as fuel level, engine
temperature, ambient temperature, ambient barometric pressure,
etc.
[0046] The reaching of one or more thresholds during the EONV test
may be indicative of an intact fuel system. Not reaching a
threshold during the EONV test may be indicative of a leak in a
fuel system. However, based on operating conditions, not reaching a
threshold during the EONV test may be considered an inconclusive
result. The comparison(s) of resulting pressures to threshold
pressures may be extracted and stored as fuel system pressure
features at controller 12.
[0047] Continuing at 242, method 200 may include comparing an
observed fuel system vapor pressure to an expected fuel system
vapor pressure based on an Antoine equation relationship. The
Antoine equation describes the relationship between pressure and
temperature of a vapor in a sealed setting. A linear relationship
between pressure and temperature may be derived when pressure is
transformed via a log operation and temperature is transformed via
an inverse operation. A flow-chart for an example high-level method
300 for comparing observed vapor pressure to expected vapor
pressure based on an Antoine equation relationship is shown in FIG.
3A. Method 300 may be carried out by a controller, such as
controller 12, and may be stored as executable instructions in
non-transitory memory.
[0048] Method 300 may begin at 305. At 305, method 300 may include
estimating fuel system vapor pressure based on current operating
conditions. Using the Antoine equation, a linear relationship may
be described between vapor pressure and temperature. In general,
the Antoine equation may be expressed as:
log P=A-(B/(C+T))
Where P is pressure, T is temperature (in degrees Fahrenheit) and
A, B, and C are fuel composition specific coefficients. The fuel
composition specific coefficients may be based on the properties of
a fuel stored in the fuel tank. Temperature may be inferred or
measured, such as by fuel tank temperature sensor 121. FIG. 3B
shows an example plot 325 showing the linear relationship between
log P and 1/T for two different fuel compositions. Line 330
represents a pressure/temperature relationship for E100 (100%
ethanol), while line 335 represents a pressure/temperature
relationship for E0 (100% petrol). Other blends of ethanol/petrol,
such as E10, E15, E85, etc. may have relationships with slopes
falling between the slopes of line 330 and line 335. Similar
relationships may be derived for other fuels, such as diesel fuel,
liquid natural gas (LNG), etc. An estimated pressure based on
temperature and fuel composition may be further based on fuel fill
level, fuel tank configuration, etc. The estimated pressure may be
considered the pressure at equilibrium.
[0049] Continuing at 310, method 300 may include determining the
observed fuel system pressure at pressure equilibrium. For example,
following sealing of the fuel system (e.g., closing of the CVV) the
fuel system pressure may be monitored until the pressure reaches
equilibrium. For example, the fuel system pressure may be measured
at regular intervals, until two or more consecutive measurements
are within a threshold of each other. Continuing at 315, method 300
may include determining a minimum magnitude of error between the
observed equilibrium pressure and the estimated pressure of the
fuel system. The minimum error may then be extracted as a feature
for classification.
[0050] For example, FIG. 3C shows an example timeline 350 for an
EONV test wherein a minimum magnitude of error between an observed
equilibrium pressure and an estimated pressure of the fuel system.
Timeline 350 includes plot 360, indicating a status of a canister
vent valve (CVV) over time. Timeline 350 further includes plot 370,
indicating a fuel system pressure over time. Line 375 represents an
estimated fuel system pressure based on a fuel system temperature
utilizing the Antoine equation.
[0051] At time t.sub.0, a vehicle shut-off event occurs (not
shown). The CVV is open, as indicated by plot 360, and the fuel
system pressure is relatively equivalent to atmospheric pressure,
as indicated by plot 370. At time t.sub.1, conditions for the EONV
test are met, and the CVV is closed. The fuel system pressure
increases from time t.sub.1 to time t.sub.2, as heat is rejected
into the fuel system from the engine. At time t.sub.2, the fuel
system pressure reaches equilibrium. The magnitude of the
difference between the equilibrium pressure and the estimated fuel
system pressure is shown at 377. The magnitude of the difference
may then be extracted and used for classification. With the fuel
system pressure reaching equilibrium at time t.sub.2, the CVV is
opened, and the fuel system pressure returns to atmospheric
pressure.
[0052] Returning to FIG. 2, at 243, method 200 may include
determining pressure change rate distribution content for the
sealed fuel system. As the fuel system pressure reaches
equilibrium, the pressure change rate approaches 0 (.DELTA.P=0).
From the pressure change rates, a probability distribution curve
may be extracted. For an intact fuel system, the distribution
content approaching .DELTA.P=0 is stable (low standard deviation).
However, for a leaky fuel system, the distribution content is
unstable (high standard deviation). The distribution content of the
pressure change rate may be extracted for classification. In some
examples, the distribution content may be continuously extracted
using a low-pass filter without requiring buffering of the pressure
data.
[0053] FIG. 4 shows an example method 400 for determining pressure
change rate distribution content for a sealed fuel system. Method
400 may be carried out by a controller, such as controller 12, and
may be stored as executable instructions in non-transitory
memory.
[0054] Method 400 may begin at 405. At 405, method 400 may include
monitoring pressure distribution content while the fuel system is
sealed. A rate of change of fuel system pressure may be extracted
based on the fuel system pressure date over time. In particular, as
the pressure change rate approaches 0, the pressure distribution
content may be monitored and extracted by controller 12. Rates of
change may be monitored and recorded periodically. Change rates may
be determined for discrete periods of time (e.g. every 1 second)
and may be determined for overlapping time frames (e.g. a 1 second
window beginning every 0.1 seconds).
[0055] Continuing at 410, method 400 may include partitioning a
signal space into non-overlapping bins. For example, for a range of
pressure change rates, a series of non-overlapping bins may be
portioned. The non-overlapping bins may encompass a same range of
pressure change rate values (e.g. 0-1, 1-2, 2-3) a log range of
pressure change values (e.g. 1-10, 11-100, 1001-1000), or any other
suitable means of distributing incoming data. Continuing at 415,
method 400 may include initializing relative frequency values (RFV)
as zero for each non-overlapping bin.
[0056] Continuing at 420, method 400 may include, for each incoming
signal (pressure change rate), updating the bin encompassing the
incoming signal value. For example, the frequency X(i) for a bin
may be updated to equal (1-.alpha.)*X(i)+.alpha.*1; where a defines
a learning rate of constructing a probability distribution
frequency, such that captured content is effectively given a moving
window size of 1/.alpha.. Continuing at 425, method 400 may
include, for each incoming signal (pressure change rate), updating
the bins not encompassing the incoming signal value. For example,
the frequency X(i) for a bin may be updated to equal
(1-.alpha.)*X(i)+.alpha.*0.
[0057] Continuing at 430, method 400 may include updating a
probability distribution frequency vector based on the updated
relative frequency values. For example, the vector PDF may be set
equal to RFV/.SIGMA.(RFV). In this way, when a probability density
frequency is extracted, the RFV vector can be normalized as the sum
of its elements, yielding a probability density frequency vector
with a magnitude of 1.
[0058] Using a low pass filter method of extracting probability
distribution frequency allows for a computationally efficient means
of extracting a feature which does not require buffering of
previously collected data. In this way, the distribution content is
always available as a classifiable feature for determining the
status of the fuel system. Further, the learning rate may be
calibrated and updated for the specific system wherein this method
is implemented.
[0059] Returning to FIG. 2, at 245, method 200 may include
unsealing the fuel system. The fuel system may be unsealed when a
predetermined number of fuel system pressure features have been
extracted. Unsealing the fuel system may include opening the
canister vent valve while maintaining the CPV and FTIV (where
included) closed. In this way, the fuel system pressure may be
equilibrated to atmospheric pressure.
[0060] Continuing at 250, method 200 may include classifying the
extracted features using a linear classifier. For example, a
support vector machine (SVM) or tool such as Fisher's Linear
Discriminant (FLD) may be utilized to classify the extracted
features. In this way, rather than simple thresholding of test
results, multiple features, such as those described herein may
collectively indicate whether a fuel system is intact or leaking.
In some examples, an individual feature may have a large overlap
between categories (e.g. similar results for intact and leaking
systems). By combining multiple features, the class margins may be
maximized while model complexity is reduced.
[0061] An example plot 500 of classified data sets representative
of intact fuel systems and leaky fuel systems is shown in FIG. 5.
Plot 500 shows example classified data sets from intact fuel
systems (502, circles) and leaky fuel systems (503, squares). For
simplicity, the classified data sets are shown based on two feature
sets: feature set #1, shown along axis 505 and feature set #2,
shown along axis 510. In some embodiments, more feature sets may be
used, and/or multiple feature sets may be dimensionally reduced
into a single feature set.
[0062] Plot 500 includes decision boundary 515. In this example,
classified data sets falling between decision boundary 515 and axis
505 are considered indicative of leaky fuel systems, while
classified data sets falling between decision boundary 515 and axis
510 are considered indicative of intact fuel systems. Plot 500
further includes buffer region 520. Buffer region 520 includes
values within a threshold of decision boundary 515 (both towards
axis 505 and towards axis 510). In some embodiments, classified
data sets falling within buffer region 520 may be considered
indeterminate.
[0063] Returning to FIG. 2, at 255, method 200 may include
determining whether the classified features are within a threshold
of a decision boundary. As shown in FIG. 5, a value within buffer
region 520 may be indeterminate as to whether the fuel system
pressure characteristics derived during the EONV test are
representative of an intact fuel system or a fuel system with a
leak greater than a threshold. If the classified features are
within a threshold of the decision boundary, method 200 may proceed
to 215. At 215, method 200 may include recording that an EONV test
was not executed, and may further include setting a flag to retry
the EONV test at the next detected vehicle-off event. Method 200
may then end.
[0064] If the classified features are not within a threshold of the
decision boundary, method 200 may proceed to 260. At 260, method
200 may include determining whether the classified features are
indicative of a fuel system leak. For example, as shown in FIG. 5,
classified features above the decision boundary are indicative of
an intact fuel system, while classified features below the decision
boundary are indicative of a fuel system leak. If the classified
features are indicative of a fuel system leak, method 200 may
proceed to 265. At 265, method 200 may include indicating a fuel
system leak. For example, a diagnostic code may be set at
controller 12. Method 200 may then end. If the classified features
are not indicative of a fuel system leak, method 200 may proceed to
270. At 270, method 200 may include indicating the fuel system is
intact. For example, a passing test result may be recorded at
controller 12. A flag for follow up may not be set. Method 200 may
then end.
[0065] Extracting a plurality of features during an EONV test and
then classifying those features as indicative of an intact or leaky
fuel system may increase the accuracy and robustness of leak
detection. Further, entry conditions for EONV tests may be relaxed.
For example, entry conditions for barometric pressure, fuel fill
level, ambient temperature, and intake air mass may be relaxed or
eliminated. In this way, the initiation rate for the EONV test may
be increased. As described with regard to FIG. 2, initial pressure
rise and results within a threshold of a decision boundary may
effectively be used as entry conditions. This approach may increase
the execution rate of the EONV test while maintaining or improving
the leak detection rate and reducing the misclassification rate.
Further, the total execution time may be reduced, as the initial
pressure rise portion of the EONV test may provide enough
extractable features to classify a fuel system as intact or leaky.
This, in turn may reduce the amount of battery power used in
performing the test.
[0066] The systems described herein and depicted in FIG. 1 along
with the methods described herein and depicted in FIGS. 2, 3A, and
4A may enable one or more systems and one or more methods. In one
example, a method for a fuel system comprises indicating a leak in
the fuel system based on a pressure change rate distribution during
a first condition including a sealed fuel system and a pressure
change rate within a threshold of zero. The method may further
comprise applying a low-pass filter to the pressure change rate
distribution; and then indicating the leak in the fuel system based
on the filtered pressure change rate distribution. In some
examples, the method may further comprise indicating the leak in
the fuel system based on a minimum error between a measured fuel
tank pressure and a predicted fuel tank pressure. The measured fuel
tank pressure may comprise an equilibrium fuel tank pressure. The
predicted fuel tank pressure may be based on a fuel tank
temperature. The predicted fuel tank pressure may be based on a
fuel composition. The predicted fuel tank pressure may be
determined using an Antoine equation relationship. In some
examples, the method may further comprise classifying the pressure
change rate distribution and minimum error using a linear
classifier; and indicating the leak in the fuel system based on an
output of the linear classifier. The method may further comprise
indicating an indeterminate result if the output of the linear
classifier is within a threshold of a decision boundary. The
technical result of implementing this method is an increased
execution rate of an engine-off natural vacuum test. By indicating
a leak based on a pressure change rate distribution rather than
through simple thresholding, an engine-off natural vacuum test may
be performed in a greater range of conditions. In this way, entry
conditions for an engine-off natural vacuum test may be relaxed,
while maintaining or improving the leak detection rate and reducing
the misclassification rate.
[0067] A method for a fuel system comprises sealing a fuel system;
and executing an evaporative emissions leak test only if a duration
of a fuel system pressure increase event is greater than a
threshold. Executing an evaporative emissions leak test may further
comprise extracting one or more fuel system pressure features for
classification via a linear classifier; and indicating a fuel
system leak based on the extracted fuel system pressure features.
The one or more fuel system pressure features may include a minimum
error between a measured fuel tank pressure and a predicted fuel
tank pressure. The measured fuel tank pressure may comprise an
equilibrium fuel tank pressure. The predicted fuel tank pressure
may be based on a fuel tank temperature. The predicted fuel tank
pressure may be based on a fuel composition. The predicted fuel
tank pressure may be determined using an Antoine equation
relationship. The one or more fuel system pressure features may
include a pressure change rate distribution within the fuel system
during a condition where a pressure change rate is within a
threshold of zero. The pressure change rate distribution may be
filtered using a low-pass filter prior to classification by the
linear classifier. In some examples, the method may further
comprise comparing an output of the linear classifier to a decision
boundary; and indicating an indeterminate result if the output of
the linear classifier is within a threshold of a decision boundary.
The technical result of implementing this method is a reduction in
engine-off natural vacuum test duration. By, extracting fuel system
features for classification by a linear classifier the EONV test
may provide enough extractable features to classify a fuel system
as intact or leaky without running the test to a vacuum threshold
endpoint. This, in turn may reduce the amount of battery power used
in performing the test.
[0068] In yet another example, a fuel system for a vehicle,
comprising a fuel tank coupled to an evaporative emissions system;
a valve coupled between the fuel tank and atmosphere; a pressure
sensor coupled between the fuel tank and the valve; and a
controller holding executable instructions in non-transitory
memory, that when executed, cause the controller to: close the
valve; execute an evaporative emissions leak test responsive to a
duration of a fuel system pressure increase event being greater
than a threshold; determine a minimum error between an equilibrium
fuel tank pressure and a predicted fuel tank pressure; determine a
pressure change rate distribution within the fuel system during a
condition where a pressure change rate is within a threshold of
zero; using a linear classifier, indicate a fuel system leak based
on the determined minimum error and the determined pressure change
rate distribution; and indicate an indeterminate result if an
output of the linear classifier is within a threshold of a decision
boundary. The technical result of implementing this system is an
increase in the robustness of an engine-off natural vacuum test. By
evaluating the initial fuel system pressure increase event, the
misclassification rate of an engine-off natural vacuum test may be
reduced. In this way, false results can be avoided, saving
unnecessary warranty inspections and reducing overall costs for the
vehicle manufacturer.
[0069] Note that the example control and estimation routines
included herein can be used with various engine and/or vehicle
system configurations. The control methods and routines disclosed
herein may be stored as executable instructions in non-transitory
memory. The specific routines described herein may represent one or
more of any number of processing strategies such as event-driven,
interrupt-driven, multi-tasking, multi-threading, and the like. As
such, various actions, operations, and/or functions illustrated may
be performed in the sequence illustrated, in parallel, or in some
cases omitted. Likewise, the order of processing is not necessarily
required to achieve the features and advantages of the example
embodiments described herein, but is provided for ease of
illustration and description. One or more of the illustrated
actions, operations and/or functions may be repeatedly performed
depending on the particular strategy being used. Further, the
described actions, operations and/or functions may graphically
represent code to be programmed into non-transitory memory of the
computer readable storage medium in the engine control system.
[0070] It will be appreciated that the configurations and routines
disclosed herein are exemplary in nature, and that these specific
embodiments are not to be considered in a limiting sense, because
numerous variations are possible. For example, the above technology
can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine
types. The subject matter of the present disclosure includes all
novel and non-obvious combinations and sub-combinations of the
various systems and configurations, and other features, functions,
and/or properties disclosed herein.
[0071] The following claims particularly point out certain
combinations and sub-combinations regarded as novel and
non-obvious. These claims may refer to "an" element or "a first"
element or the equivalent thereof. Such claims should be understood
to include incorporation of one or more such elements, neither
requiring nor excluding two or more such elements. Other
combinations and sub-combinations of the disclosed features,
functions, elements, and/or properties may be claimed through
amendment of the present claims or through presentation of new
claims in this or a related application. Such claims, whether
broader, narrower, equal, or different in scope to the original
claims, also are regarded as included within the subject matter of
the present disclosure.
* * * * *